Light sensitive alarm system



June 22, 1965 5, cow 3,191,048

LIGHT SENSITIVE ALARM SYSTEM Filed Nov. 22. 1961 FREE RUNNING INVENTOR. 3? RICHARD GEORGE COWEN United States Patent 3,191,048 LIGHT SENSITIVE ALARM SYSTEM Richard G. Cowen, Frederick, Md, assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Nov. 22, 1961, Ser. No. 154,374 13 (llaims. (Cl. 250-221) This-invention relates to an alarm system and more particularly relates to an alarm system using a plurality of pairs of photoresistors at spaced points about a room or other area which it is desired to protect.

In particular, the invention provides an alarm system in which the photoresistors of any one pair are connected in diagonally opposite legs of a bridge circuit to provide a rapid change in voltage across the bridge when the light pattern at that pair is changed as by the presence of an intruder. By connecting these photoresistors in diag onally opposite legs of the bridge, a voltage change effect is produced which is the combined effect of both photoresistors and has a trigger action so that only a single light source is required which is common to all of the pairs of photoresistors.

Burglar and other alarm systems are well known in the art. They include both simple and complex systems and the use of one or the other frequently depends upon the value of the material or facility desired to be protected. One problem in this field is that a good alarm system often costs the same amount of money as the value of the article desired to be protected. In other in stances where secret documents are desired to be protected, the value of the documents is an intangible so that the cost of the alarm system may outweigh the value of the documents. Accordingly, there has long been a need for an inexpensive alarm system which can protect large areas.

One method of providing an alarm system for a room has been the use of photoelectric systems at each entrance to the room. At each entrance, at one side of the doorway, for example, there is a light source and a lens; on the other side of the doorway there is another lens and a photoelectric cell so that the light beam is focused upon the photoelectric tube and is interrupted when an individual passes through the doorway. It is obvious that such a system is expensive, since it requires a plurality of light sources and lenses and in addition such a system does not protect the room should entry be obtained through other than the usual entrance ways, as for example a window.

This invention has overcome these disadvantages While yet producing much greater protection at a fraction of the cost of previous systems. In particular, the invention provides a single light source which is positioned in the ceiling or some other central location to scatter light energy through the room. A plurality of pairs of photoresistors are positioned throughout the room and are connected into a bridge circuit. An alarm relay is connected across the bridge. Each pair of photoresistors has one photoresistor in each of two diagonally opposite legs of the bridge so that should the light pattern at any one position suddenly change, each of the photoresistors at that position will tend to operate the relay and their combined efiect positively does operate the relay. In the absence of an intruder, the light is equally distributed or otherwise balanced at the photoresistors so that the bridge circuit is balanced and the alarm inoperative.

Since the room which it is desired to protect may be at a remote location where AC. power is not readily available and since a prospective burglar is likely to cut the line having such power, it is desirable to have a battery source for energizing the lamp. However, if the 3,l9l,4=8 Patented June 22, 1965 ice cation is remote, frequent checks would have to be made to determine the battery condition. An automatic battery charging scheme could of course be devised but this again would be quite expensive. Accordingly, this system provides as an additional aspect of this invention, a battery supply for the lamp which is alternately interrupted so that the lamp is cyclically energized for a brief period of time and then denergized for a longer period of time. Thus the battery life is prolonged not only because it is supplying power for much shorter time periods, but also because it can recharge itself during its off period. In the previous systems the lamp was generally continually energized since an opening of the battery supply would be the equivalent of breaking the light beam by an intruder thus sounding an alarm. However, in this invention with all of the photoresistors in the bridge circuit and with one of the photoresistors of one pair in one leg of the bridge and the other photoresistor of the same pair in the diagonally opposite leg, during oil period of the battery, all of the photoresistors of the bridge are equally affected by any fluctuation of the light source itself so that no alarm is sounded and battery power is saved.

Accordingly, it is an object of this invention to provide an improved alarm system.

It is another object of this invention to provide an alarm system using at least one pair of photoresistors positioned at one location and connected into diagonally opposite legs of a bridge circuit.

It is another object to provide an alarm system using a single light source and a plurality of pairs of photoresistors positioned about an area to be protected.

A still further object of the invention is to provide a single light source centrally located within a room and a plurality of pairs of photoresistors spaced about the room and connected into a bridge circuit.

A further object is to provide such system with a battery source for energizing the lamp and particularly one which is alternately on and 0E.

The above and further objects will be apparent upon reading the following detailed disclosure of which:

FIG. 1 is a pictorial diagram of a room having a light source in the ceiling and pairs of photoresistors positioned along the walls of the room to receive both direct and reflected light from the source.

FIG. 2 is a schematic diagram of a first embodiment of my invention.

FIG. 3 is a schematic diagram of a second embodiment of my invention.

FIG. 4 is a schematic diagram of another embodiment of the invention.

FIG. 1 shows a room having a single light source in the ceiling and a plurality of pairs of photoresistor 2 mounted on the Walls of the room. The light source 1 may be an incandescent or phosphorescent or any other light source energized from an AC. or D.C. source. Each pair of photoresistors has two cadmium sulphide cells 3 and 4, the connection of which is shown in FIG. 2. The mounting of the light source and photoresistors is shown for illustrative purposes only, since other arrangements are contemplated within the scope of this invention to provide a substantially equal distribution of light (both directed and reflected) to each pair of photoresistors.

FIG. 2 is a schematic drawing showing the connection of the photoresistors and the alarm circuitry. Terminals 1t and 11 are provided for connection to a Volt AC. source.

A full wave bridge rectifier 12 includes diodes 13-16. When terminal 10 is positive, condenser 17 is charged through resistor 18 and diodes 13 and 16 so that the voltage on line 2% is a positive D.C. voltage with respect to line 19. On the other half of the AC. wave, condenser 1'? is again charged with the same polarity. Resistor 18 may of course be a choke if additional filtering is desired.

Resistor 21 is a bleeder resistor to bleed oil the charge from condenser 17 when the apparatus is disconnected from the power source. Potentiometer 22 acts as a DC. voltage divider so that the voltage on line. 23 may be adjusted as desired.

Alarm relay 24 is connected between lines 23and 2t and is normally energized. Should the D.C. power supother pair are shown as 32 and 33. Thus the pair of I photoresistors on one side of the room has one photoresistor in one leg of a bridge circuit and its other photoresistor is in the diagonally opposite leg of the bridge circuit; similarly photoresistors 32 and 33 are in diagonally opposite legs of the bridge. 1

Assuming for example that the voltage on line 23 is a plus 50 volts and that the lamp 1 in FIG. 1 is energized, the voltage at both points 35 and 36 is approximately volts so that relay 34 is deenergized. Now if a person enters the room near the photoresistor pair 35 and 31, there will be less light on those resistors so that their resistances increase. Accordingly, the voltage at terminal 35 may drop to 15 volts; if junction 35 had stayed at 25 volts, a ten volt difference would have been availableto operate the relay 34. However, since photoresistor 31 is in the same pair as 31 and views the same scene, its resistance has also increased so that the voltage at junction 36 rises to 35 volts. 'Thus 20, volts rather than '10 volts are available for operating relay 34.

Therefore this bridge connection of the photoresistors of the various pairs provides a large voltage change to operate the relay. Also because this large voltage is present, a high impedance relay coil may be used at 34 to reduce the amount of steady state curent flowing through the relay.

Not only are the above mentioned advantages available, but in addition, it will be noted that this system permits .the use of both a' constant or fluctuating light source since a fluctuating light source will have an equal effect on all of the legs of the bridge and thus maintain the same equal voltage at 35 and 36 (25 volts in the assumed example). V

Operationof relay 34 closes contacts 37 to operate a second alarm relay 38 to indicate the presence of an intruder. 7 set means (not shown) are used 'with such a relay as is conventional. V

In addition, the alarm circuitry may be modified by connecting line 29 on line 6 5 to line 19 and line 23 on line 66 toline 2G or 23 (if source 26 is a battery);

With the above modification source 26 will be con- I nected as an auxiliary power supply so that should the main power supply fail and relay 24 be deenergized, source 26 will supply DC. power between lines 23 and 19 over lines 65 and' d; substitution of the auxiliary power supply will of course energize relay 2410 thus disconnect this auxiliary power supply from lines 23 and 19. Therefore, in this suggested modification, the auxiliary power supply 26, relay 24, and alarm 27 will be alternately energized or operative in a pulsating manner. This chopped D.C. auxiliary power on lines 23 and 19 will still operate the bridge of photoresistors since in the manner suggested above, this photoresistor bridge Suitable holding contacts (not shown) and recircuit is independent of voltage and transient light variations.

FIG. 3 shows a second embodiment of the invention showing a complete battery operated device. A free running multivibrator is shown at and has its emitters or cathodes connected to line 41. One anode or collector is connected through resistor 46 to the positive source on line 42. The other anode is connected through lamp 47 to line 42. The multivibrator continually switches from conduction in one tube or transistor to conduction in the other as is well known. The arrow 48 is shown to indicate that the MN. is adjustable so that two transistors are'conducting for different time periods within a switching 'cycle and that the repetition rate of the cycle is also adjustable. For example, M.V. 45 may be adjusted so' that the right side of the M.V. is conducting for only ten percent of a cycle, and the left hand is conducting for the remaining 90 percent of the cycle; Thus lamp 47 will be intermittedly energized and result in'a conservation of battery power since lamp 47 would draw more current than resistor 46. Alternatively as shown in FIG. 4 the M.V. may control a relay to close the battery circuit to the photoresistor bridge, through switch 61 for short periods of time.

The battery power then connects over lines 41 and 42 to a bridge type network of photoresistors connected in the same manner as in FIG. 2 with the exception that four pairs of photoresistors are shown in FIGS. 3 and 4. When relay 34 is energized, the contacts are closed at 49 and alarm 56 is energized. .Thus even though the light a source in FIG. 3 is fluctuating on and off, the bridge is balanced and relay 34 does not operate until an intruder enters the room.

Having thus described the preferred form of my invention,'it will be obvious to those skilled in the art that there are equivalent elements and circuits usable with my invention. For example, while I use an RCA 7163 Cadmium Sulphide photoresistor, other devices including semi-conductors may be used. However, the cadmium sulphide cell is very sensitive to variations in light from a light source (could be dark light) removed some distance from the cell and without the benefit of a'lens system.

This photoresistor has a logarithmic response so that large variations in resistivity occur for slight variations in light. Thus the voltage variations at 35 and 36 in FIG. 2 will be much greater than suggested.

In addition, it should be noted that in the absence of an intruder, light is received by all of the photoresistors so that the bridge circuitisa relatively small impedance between lines 19 and 23 in FIG. 2. 'By providing that potentiometer 22 has a large impedance, the current flow for energizing relay 24 must flow throughthe photoresistor bridge circuits.

. Now if the light source in FIG. 2 were to suddenly fail, the impedance of the bridge circuit would rise to thereby deenergize relay 24 and operate alarm 2'7.

Thus relay 24 serves a dual function of indicating both a voltage and light source failure.

Therefore, my invention is described in the following claims:

I claim: a r 1. In an alarm circuit having a four terminal Wheatstone bridge network, the improvement comprising a single source of light energy, a plurality of pairs of similar light sensitive means in which the members of each .pair are'physically adjacenteach other and in which the pairs are physically spaced from each other a sub-v stantial distance, a direct current source connected across the two input terminals of said bridge, current responsive means connected across the two output terminals of said bridge, means for connecting a light'sensitive member of one pair in one leg of the bridge and the other light sensitive member of said one pair in a leg of the bridge diagonally opposite said one leg, means for similarly connecting the individual light sensitive members of another pair in the other diagonally opposite legs of the bridge so that there are light-sensitive members in all four legs of the bridge, said pairs being physically positioned for receiving substantially equal light energy from said light source whereby the bridge remains balanced for variations of both the voltage and light source and is unbalanced by interruption of the light intensity at one pair.

2. A combination as in claim 1 in which said light sensitive means includes a photoresistor.

3. A combination as in claim 1 in which said light sensitive means incudes a cadium sulphide cell.

4. A combination as in claim .1 and further including means for alternately energizing and deenergizing said light source.

5. A combination as in claim .1 and further including an auxiliary power supply, and means responsive to failure of said direct current source for substituting said auxiliary supply for said direct current supply, said latter means including means for alternately connecting and disconnecting said auxiliary power supply when substituted.

6. An alarm circuit for protecting an enclosure including a four terminal Wheatstone bridge circuit comprising a direct current source of power connected across the two input terminals, a single light source, means for alternately energizing and deenergizing said light source from said power source, a plurality of pairs of light sensitive means, the individual light sensitive members of one pair being connected in diagonally opposite legs of the bridge and the individual light sensitive members of another pair being connected in the other diagonally opposite legs of the bridge, both light sensitive means of any pair having substantially the same location within the enclosure and each pair being positioned at different positions throughout the enclosure, and alarm means connected across the two output terminals of said bridge whereby the bridge is balanced during both the energization and deenergization of said light source and said alarm is operated only by interruption of the light beam at one pair of said light sensitive means.

7. A combination as in claim 6 in which said alarm means includes a relay and relay contacts for closing an alarm circuit.

8. A combination as in claim 7 further including a free running multivibrator adjusted to energize said light source for a relatively short time and deenergize it for a relatively long time.

9. In an alarm system using a light source and a voltage source both of which control photoresistors for detecting the presence of intruders, the improved combination comprising a plurality of pairs of photoresistors in a bridge circuit across said voltage source with at least one photoresistor in each leg of the bridge and with one of the photoresistors of one pair connected into one leg of the bridge and with the other photoresistor of said one pair connected into a leg of the bridge diagonally opposite said one leg and with each of the photoresistors of the other pair similarly connected into the remaining diagonally opposite legs of the bridge, a cyclically varying light for illuminating said pairs, and means responsive to an unbalance in the bridge circuit whereby the bridge remains balanced for fluctuations in either the voltage or light source and is unbalanced only by interruption of the light at a pair.

19. The combination as in claim 9 further including a single relay and means connecting said relay in series between the voltage source and said bridge circuit .for indicating a failure in either the light or voltage source.

11. In an alarm system having a light source, a voltage source and a bridge circuit with photoresistors in which the bridge circuit is connected across the voltage source and the light source controls the resistivity of the photoresist-ors, the improvement in the combination including a single relay connected in series between said bridge circuit and said voltage source such that current fiows through the bridge and relay from said voltage source, said bridge circuit including photoresistors in each arm of bridge and a relatively large impedance in shunt With said voltage source and having a tap connected to the junction of the relay with said bridge circuit, whereby the relay is normally energized in the presence of both the voltage and light source but a failure of either source results in deenergization of the relay to indicate a fault.

12. In an alarm system having a Wheatstone bridge circuit with two input terminals and two output terminals, the improved combination comprising a single source of light energy, a plurality of pairs of light sensitive members, a direct current source connected across the input terminals of the bridge, means for connecting the individual light-sensitive members of one pair in diagonally opposite legs of the bridge, means for similarly connecting the individual light sensitive members of another pair in the other diagonally opposite legs of the bridge so that there are light sensitive members in all four legs of the bridge, current responsive means connected across the output terminals of said bridge, and means for alternately connecting and disconnecting said direct current source to said light sensitive bridge circuit.

13. In an alarm system having a Wheatstone bridge circuit with two input terminals and two output terminals, the improved combination comprising a single source of light energy, a plurality of pairs of light sensitive members, a direct current source connected across the two input terminals of said bridge, means .for connecting the individual light-sensitive members of one pair in diagonally opposite legs of the bridge, means for similarly connecting the individual light sensitive members of another pair in the other diagonally opposite legs of the bridge so that there are light-sensitive members in all four legs of the bridge, current responsive means connected across the output terminals of said bridge, a tree running multivibrator, and means connecting said multivibrat-or for alternately energizing and deenergizing said light source.

References Qited by the Examiner UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner.

WALTER STOLWEIN, Examiner. 

1. IN AN ALARM CIRCUIT HAVING A FOUR TERMINAL WHEATSTONE BRIDGE NETWORK, THE IMPROVEMENT COMPRISING A SINGLE SOURCE OF LIGHT ENERGY, A PLURALITY OF PAIRS OF SIMILAR LIGHT SENSITIVE MEANS IN WHICH THE MEMBERS OF EACH PAIR ARE PHYSICALLY ADJACENT EACH OTHER AND IN WHICH THE PAIRS ARE PHYSICALLY SPACED FROM EACH OTHER A SUBSTANTIAL DISTANCE, A DIRECT CURRENT SOURCE CONNECTED ACROSS THE TWO INPUT TERMINALS OF SAID BRIDGE, CURRENT RESPONSIVE MEANS CONNECTED ACROSS THE TWO OUTPUT TERMINALS OF SAID BRIDGE, MEANS FOR CONNECTING A LIGHT SENSITIVE MEMBER OF ONE PAIR IN ONE LEG OF THE BRIDGE AND THE OTHER LIGHT SENSITIVE MEMBER OF SAID ONE PAIR IN A LEG OF THE BRIDGE DIAGONALLY OPPOSITE SAID ONE LEG, MEANS FOR SIMILARLY CONNECTING THE INDIVIDUAL LIGHT SENSITIVE MEMBERS OF ANOTHER PAIR IN THE OTHER DIAGONALLY OPPOSITE LEGS OF THE BRIDGE SO THAT THERE ARE LIGHT-SENSITIVE MEMBERS IN ALL FOUR LEGS OF THE BRIDGE, SAID PAIRS BEING PHYSICALLY POSITIONED FOR RECEIVING SUBSTANTIALLY EQUAL LIGHT ENERGY FROM SAID LIGHT SOURCE WHEREBY THE BRIDGE REMAINS BALANCED FOR VARIATIONS OF BOTH THE VOLTAGE AND LIGHT SOURCE AND IS UNBALANCED BY INTERRUPTION OF THE LIGHT INTENSITY AT ONE PAIR. 